Conventional single phase AC motors typically include a starting circuit which must be switched into operation in order to initially start the AC motor and must thereafter be switched out of operation as the AC motor approaches operational speed. For this purpose, the main winding of the AC motor is connected in parallel with a starting circuit which includes a series-connected start winding and start capacitor. A switch is included in the starting circuitry in series with the start capacitor and the start winding in order to operably connect and disconnect the start winding and start capacitor at the appropriate times during operation. For this purpose, various mechanical and electro-mechanical switches have been employed to operably connect and disconnect the start winding and start capacitor.
One of the more common types of starting switches currently in use is a mechanical centrifugal switch mechanism which senses the centrifugal force produced by the rotation of the rotor. At a predetermined motor speed, the centrifugal switch opens to disconnect the starting circuitry. In common electro-mechanical switches, current relays which utilize the current in the main winding as the control parameter have been utilized. In other electro-mechanical starting switches, a potential relay has been employed which utilizes the back EMF in the start winding as the control parameter.
The mechanical and electro-mechanical switches have the inherent problems of the failure of moving parts and worn contacts on the switch. For example, pin setting equipment in bowling alleys typically utilizes several motors which operate various mechanisms. At least one of the motors for the equipment is a single phase AC motor which must start and stop three times during each cycle of the machine. The motor typically employed for this purpose is a single phase AC motor having a centrifugal switch mechanism. Since the motor may often be started on the order of 15,000 times a day, the reliability of the centrifugal switch is a major concern. The failure of the centrifugal start switch causes a large percentage of the machine breakdowns.
More recently, solid state switches are being employed to replace the mechanical or electro-mechanical switches. Conventional solid state switches have used the magitude or phase difference in the main winding current and the start winding current as parameters to trigger the switch to disconnect the start winding. The problem with utilizing the magnitude of the winding currents as control parameters is that the winding currents are dependent upon motor design, size, component values, line voltage and motor loading. As a result, start switches utilizing winding currents as control parameters must either be adjusted or properly calibrated for specific applications. Like current magnitude, current phase is also a strong function of motor design and operating conditions.
Solid state switches have not been very reliable in various applications. For example, temperature variation, unusual motor loading, and marginal components may cause false triggering, erratic operation, or even failure.
Another problem which may be encountered in the use of conventional start switches for disconnecting the start winding is possible interference with the operation of an electronic brake employed on certain AC motors. Conventionally, when a dynamic brake is operated, the main winding of the motor is shorted out by placing the start capacitor in series with the main winding. If the start switch senses a parameter which causes the switch to close during the time that the brake is being applied, the effectiveness of the brake is greatly decreased.